1. Technical Field
The present invention relates to a color killer circuit that masks a color component when a composite video signal including a color component signal and a luminance component signal is processed.
2. Related Art
In video signals for the NTSC system or other systems (composite video signals), as shown in FIGS. 9 and 10, one field of a video signal SIN includes a vertical synchronization period TV, a horizontal blanking period TH, and a video signal period TS, and the video signal SIN for one horizontal line includes a horizontal blanking period TH, a color burst period TC, and an effective video signal period TI. It should be noted that, for the sake of clarity of description, FIGS. 9 and 10 show an example in which the ratios of the respective periods are changed from those of an actual video signal SIN.
In the processing of such a video signal to display a video image, when the strength of an input video signal is weak, or when noise is large with respect to the strength of the video signal, routinely processing the video signal may result in occurrence of uneven color or other irregularities, causing a video image to distort. With this being the situation, a color component output may be blocked so that a monochrome video image containing luminance information only is displayed.
In order to perform processing while thus masking a color component, a color killer circuit has been used. The color killer circuit includes, as shown in FIG. 11, an analog-to-digital converter 10, a burst amplitude detection unit 12, a noise detection unit 14, and an OR element 16.
The analog-to-digital converter 10 converts an input video signal to a digital signal, which is input to the burst amplitude detection unit 12 and the noise detection unit 14. The analog-to-digital converter 10 samples signals at a sampling rate four times a frequency of the color burst included in the video signal. In this process, as shown in FIG. 12, sampling timing is adjusted so that a sampling point matches with a peak of one cycle of the color burst.
The burst amplitude detection unit 12 receives the digitalized video signal, and extracts an amplitude of the color burst for each video signal of one horizontal line. The burst amplitude detection unit 12 determines whether or not the extracted amplitude is less than a preset first threshold value. An output to the OR element 16 is set to HIGH level when the extracted amplitude is less than the preset first threshold value, and otherwise is set to LOW level.
The noise detection unit 14 receives the digitalized video signal, and determines a strength of noise superimposed on the video signal. The strength of noise can be estimated based on whether or not an average strength of input video signals is less than a preset second threshold value. The noise detection unit 14 sets an output to the OR element 16 to HIGH level when an average strength of input video signals is less than the preset second threshold value, and otherwise sets it to LOW level.
The OR element 16 receives outputs from the burst amplitude detection unit 12 and the noise detection unit 14, sets a killer determination signal to HIGH level when at least one of those outputs has HIGH level, and sets a killer determination signal to LOW level when both of those outputs have LOW level. Thus, when the strength of an input video signal is weak, or when noise is large with respect to the strength of the video signal, it is possible to determine, based on the killer determination signal, whether or not the color signal should be masked to perform processing.
However, the above-described related-art color killer circuit requires, in addition to the burst amplitude detection unit 12 for extracting a maximum peak value of the color burst, the noise detection unit 14 for determining whether or not the electric field strength of the video signal is under weak electric field conditions under which it is less than a predetermined threshold value.
Therefore, there have been problems in that the color killer circuit has a complicated circuit structure, and is large in circuit scale. As a result, there is also a problem in that the cost of manufacturing an apparatus including the color killer circuit is increased.